Normally I'd agree that a player who plans to leave should have the roster frozen to prevent bad deals, but if the roster hasnt been frozen, you need to scrutinize the deal based on its merits.
if this is a 2 keeper league, then a first round pick is the equivalent of a third round pick in a redraft. I could easily see AB going in the 2nd or 3rd round. Ditto for Kelce and Julio.
the guy inheriting his team wont do badly because of the deal. he is at the very least, getting some keepable players. this is especially true if his team was weak in terms of potential keepers.
even if it wasnt weak in terms of potential keepers(ie. he has a strong team), this deal gives him a legit shot at a title and likely represents fair value for a team trying to win it all.
In this case, I'd let the deal go through. hes acting as a normal team might even if he is leaving.
You can't keep players drafted in first 5 rounds. The first 50 picks of the Draft are not eligible to be kept. AB, Julio, and Kelce were the team's first 3 picks in our August draft.